The use of communication cables which include a plurality of optical fibers is rapidly expanding. An optical fiber cable may comprise a plurality of glass fibers--each of which is protected by at least one layer of a coating material. The optical fibers may be assembled into units in which the fibers are held together by binder ribbons or within tubes to provide a core. Another optical fiber cable core includes a ribbon type optical fiber arrangement in which a plurality, such as twelve fibers for example, are arrayed together side by side. A plurality of these ribbons may be stacked to obtain a high fiber count cable. The core is enclosed by a plastic core tube(s) and a plastic jacket. Ribbon type cable in which a relatively large number of optical fibers may be packaged appears to be ideally suited for fiber-to-the-customer use.
Whatever the structure of a cable, there must be provisions for splicing transmission media at an end of a given length of cable to corresponding transmission media at an adjacent end of another length of cable. When metallic conductors are present, it is conventional to use a splice closure, within which strength members of the cable ends are anchored and all conductors spliced, wrapped, stored and environmentally protected. Typically, during the splicing of metallic conductors, the conductors are sharply bent to provide access to other connections.
The physical nature of optical fibers precludes the adoption of splicing techniques which are used with metallic conductors within such a splice closure. Because of their small size and relative fragility, special considerations must be given to the handling of optical fibers in closures. Transmission capabilities may be impaired if an optical fiber is bent beyond an allowable bending radius, the point at which light no longer is totally contained in the core of the fiber. Furthermore, fibers are brittle and their expected lives will be reduced if bent more than the minimum bending radius. Generally, the radius to which the optical fiber can be bent without affecting orderly transmission is substantially greater than that radius at which the optical fiber will break. Whereas glass and silica, the materials used to make optical fibers, are in some respects stronger than steel, optical fibers normally do not possess this potential strength because of microscopic surface fractures, which are vulnerable to stress and spread, causing the fiber to break easily.
It should be clear that an optical fiber cable does not lend itself to the splicing practices of metallic communication conductors. Individual optical fibers cannot just be twisted, tied, wrapped and moved into a splice closure in anything like the manner of metallic conductor cables. These small-diameter glass fibers cannot be crimped or bent at small angles without breakage. Inasmuch as glass fibers have memory and tend to return to a straight-line orientation, placement in a splice closure becomes somewhat difficult. Moreover, the interconnection of optical fibers is a precision operation which in the past has tended to discourage some from performing splicing operations within a manhole, in a handhole, or a pole-suspension elevation. And yet, to do otherwise becomes more expensive.
These problems are particularly acute in multifiber cables where individual optical fibers must be spliced in a manner which allows repairs and rearrangements to be made in the future. In addition, fiber slack normally must be provided adjacent to the splices. The need to store the slack further complicates the problem of providing a suitable optical fiber closure.
When splicing optical fibers by fusion or by mechanical means, it becomes necessary to provide enough slack fiber so that the fiber can be pulled out of the splice case for the preparation of fiber ends and the joining together. This requires at least about 0.5 meter of fiber from each cable to be stored in the splice closure when the closure is sealed, that is when the splicing has been completed. For a multifiber cable there must be a method of storing this slack, of protecting the splice, and of keeping the fibers together in an orderly manner. The splices should be easily accessible to facilitate the rearrangement of the individual optical fibers and splices. Additionally, it has become common practice to loop an optical cable through the closure and access only certain fibers, ribbons or units for splicing to others. The remaining fibers (designated "express" fibers, ribbons or units) must be stored in the closure. The length of these stored transmission media may be as much as 13 feet in length.
Optical fiber connecting arrangements must be protected from forces which could distort their shape or pull the fibers out of the arrangements. Although it is important that large forces are not applied to the connective arrangements, it also is important that they be secured in position. Any axial or torsional movement thereof could cause movement of the fibers which could cause attenuation of the optical signal being transmitted therealong. As must be expected, fiber splice organizers and splice closures are available in the prior art. These prior art organizers and closures have suffered from a variety of shortcomings. Typically, they have been somewhat complex, difficult to use and difficult to access. Moreover, they do not accommodate a large enough number of splices in a compact space.
For example, U.S. Pat. No. 4,913,522 discloses an optical fiber organizer which accommodates about 8 to 10 trays, and each tray may store from 5 to 20 optical fibers. The trays are stacked, one on top of the other, and each is hinged separately at one end thereof to a carrier, thus allowing them to move relative to one another like bound pages. In a preferred embodiment, it accommodates about 100 fibers which is somewhat low in view of the projected number of fiber users in a society wired for connection to the "information superhighway."
U.S. Pat. No. 4,927,227 discloses an optical fiber cable closure in which a support member comprises a support base for supporting an optical fiber breakout and a plurality of splice trays. However, there appears to be limited storage capacity and lack of ability to accommodate as many different splicing arrangements as desired. Current thinking would require each tray to store at least thirty-six splices.
U.S. Pat. No. 5,185,845 discloses an optical fiber closure having enhanced storage capability in which each tray stores thirty-six splices, but the closure only handles about six trays which provides a total capacity of about 216 splices. In each of the above patents, trays are individually hinged at one end in a stair-step manner which is not particularly space efficient. Additionally, the stair-step design of the prior art accommodates only one tray size. Further, end-hinging plastic members are used and they must be carefully engineered to handle substantial stress in the event that the closure is dropped or when the trays are oriented vertically (e.g., when splicing work is being done in a lower tray).
What the prior art appears to lack is an optical fiber cable closure of low-cost construction which provides more efficient space utilization and a simpler, stronger hinging apparatus. Further, a compact closure which is capable of handling an increased number of splices is desirable. And finally, a closure which efficiently accommodates trays of varying thickness is needed.